Optical waveguide lasers and amplifiers, whether made of a fiber or in integrated optics, are of particular interest since they are components which can be manufactured at a relatively low cost and with techniques now well consolidated, and which are capable of operating at several of the wavelengths important for present or future applications in fields such as telecommunications, medicine, spectroscopy, sensing and the like. Considering, in particular applications, in telecommunications, waveguide lasers and amplifiers can obviously be easily coupled to other waveguide components of an optical communication system, such as the fiber forming the transmission medium, directional couplers, and the like.
An optical waveguide laser or amplifier is usually formed of a length of a monomode active guide, obtained by using suitable dopand ions, e.g. rare earth ions, and an optical pump radiation is launched into said guide to supply the energy required to produce the so-called population inversion, possibly allowing stimulated emission which the amplification effect is based. In the case of a laser, the guide is clamped in position in a resonant structure which can be schematized by a pair of mirrors placed at the guide ends, and the pump radiation is launched into the guide through one of two mirrors. In the case of an amplifier, the guide conveys both the optical signal to be amplified and the pump radiation, which are launched by means of dichroic components or directional couplers having suitable coupling factors for the two wavelengths. Some embodiments of optical fiber lasers and amplifiers are described, e.g. in the article "Review of rare earth doped fibre lasers and amplifiers" by P. Urquhart, IEE Proceedings, Vol. 135, Part J, No. 6, December 1988.
Usually, the pump radiation is directly launched into the active guide region by using sources consisting for example of semiconductor lasers, which are available at the wavelengths of interest for the applications mentioned above. In the case of monomode guides, this can represent a severe limitation. In fact, for a given power of the pump radiation, the fiber length in which the pump effect is present, and hence the attainable gain, have a determined value which cannot be exceeded. To obtain higher gains, it would be necessary to use a plurality of separate sources. Yet, as is known, it is impossible to sum in a monomode guide the radiations emitted from a plurality of independent sources having the same wavelength and the same polarization, because this would be a violation of the conservation law of radiance, as described by P. Di Vita and R. Vannucci in the paper entitled `The "Radiance Law" in Radiation Transfer Process', Applied Physics, No. 7 (1975), pages 249-255. Therefore, the maximum injectable power in a monomode guide will be that of two orthogonal polarization sources and the improvement obtainable is rather limited. The only alternative would be to use very powerful individual sources, but sources of this type are not available for all wavelength of interest.